The invention relates to an endoprosthesis with two or more individual parts, of which one has a cone part with a conical bore and of which another has a cone part with a complementary conical projection, the parts being connected by inserting the conical projection into the conical bore, and there being provided, in the area of the cone connection, a securing screw whose axis is perpendicular to the cone axis and which is screwed into one of the cone parts and with its front end engaged in a recess in the other cone part.
Endoprostheses often consist of two or more individual parts which can then be joined together as required, for example a prosthesis main part and a prosthesis head. Such multi-part prostheses have the advantage that, with relatively few individual parts, a great many combinations between prosthesis head and prosthesis main part can be produced, so that the number of parts to be kept in store is lower.
The prosthesis is made up from the two individual parts by means of the part with the conical bore being placed on the part with the conical projection and driven home. This provides a generally reliable and permanent connection, as is necessary.
It is known to secure a connection of this type, which is already inherently firm, by means of an additional securing screw (EP 0,000,549 A1).
The aim of the invention is to provide an endoprosthesis of the type mentioned at the outset, in which the two parts can be connected permanently to one another even more reliably.
The solution according to the invention consists in the front end of the securing screw being tapered and cooperating eccentrically with the recess.
By means of the eccentric cooperation, the securing screw not only secures the connection, but exerts an additional force in the direction in which the cone connection is pressed together still further. Such an eccentrically acting cone screw is known from shelf construction (FR-B-2,222,889). Apart from the fact that shelf construction can provide no stimulus for the constructor of an endoprosthesis, the cone screw there has the purpose of drawing together two parts which can be displaced one inside the other, the connection falling apart without the cone screw. In the invention, the cone connection is in itself already a reliable and firm connection, whose reliability, however, is still further increased by the arrangement of the cone screw according to the invention.
Such securing screws are normally only used in cases where the two parts would not cling to one another reliably without such screws, for example if a cylindrical bore is placed on a cylindrical support. It is the contribution of the invention to have recognized that by using a securing screw, even though the latter does not appear to be absolutely necessary in a cone connection, an even more secure attachment can be obtained.
In an advantageous embodiment, the securing screw is arranged in the outer cone part and cooperates with a bore or an annular groove in the inner cone part.
An essential feature is that the front end of the securing screw cooperates with the recess eccentrically or axially offset. If, in this case, the securing screw is arranged in the outer cone part, then the front end of the securing screw should cooperate with the recess eccentrically or axially offset to the side of the screw axis which is directed towards the tapering end. Thus, according to the invention, the securing screw contacts the outer end of one of the recess side walls with a greater force than the force exerted on the outer end of the other side wall. For example, in one embodiment, the screw contacts the outer end of the recess side wall that is closer to the narrow end of the cone parts, and does not contact the end of the recess side wall that is further from the narrow end.
In the eccentric arrangement of the invention, the securing screw not only fixes the position previously obtained by striking the parts together, but also exerts on one side a force by means of which the two cone parts are pressed together or tensioned still further. In this way, for example, tolerances can be compensated. Even in the case of very accurately machined cone parts, it is possible that the securing screw and recess will not match each other absolutely concentrically, since a small variation in dimension can lead to a greater displacement on account of the normally relatively slight slope of the cone or a normally very small cone angle in the axial direction. Such tolerances can be compensated by the eccentric arrangement. In addition, if the cone connection was made insufficiently secure by mistake, or if, as a result of aging process, the cone connection threatens to come loose, the securing screw does not prevent a renewed, firmer pressing together of the cone parts by means of external loads, since the securing screw or securing screws do not exert any force in this direction, but instead permit a movement. Thus, in the unfavorable conditions mentioned, this embodiment prevents the cone screw from having exactly the opposite effect, namely that of preventing a more secure connection.
A further advantage of this embodiment lies in the fact that the securing screw does not prevent the sintering together of the cone connection, but instead, as a result of the axially offset arrangement of the securing screw with respect to the recess, activation of the securing screw effects an even firmer pressing together of the cone connection in the longitudinal direction.
In another advantageous embodiment, the securing screw is arranged in a bore of the inner cone part provided with a thread, and cooperates with a bore or with a slot extending over a part of the periphery. The screw can in this case be turned from the outside by means of a tool, which is pushed through the outer cone part, the bore in the inner cone part being designed as a through-bore, and a counterbore or a corresponding slot being provided in the outer cone part on the radially opposite-lying side of the recess in which the screw is to be engaged. The front end of the screw can be designed to be any suitable shape. Preferably, the screw is tapered, and more preferably it is conical or frustoconical. When the front end is frustoconical, the tool engages on the flat front surface of the truncated cone--the tool thus engages on that end of the screw at which the screw is intended to exert a force on the other cone part. Among other tapered shapes, the front end of the securing screw can have concave or convex sides.
In the embodiment incorporating a securing screw having a frustoconical front end, it is essential that the securing screw cooperates with the recess eccentrically or axially offset, and namely on the side of the screw axis which is directed away from the tapering cone end. The particular advantages described above in connection with the eccentric arrangement are then once again obtained.
If the walls of the recess, with which the securing screw cooperates, are bevelled at least in the parts pointing towards the securing screw, there is no punctiform contact here, which contact could under certain circumstances lead to a deformation of the screw or the walls. Greater forces can therefore be transmitted. In this respect, the angle of the bevelling should correspond as closely as possible to the corresponding angle of the front end of the securing screw.
Instead of screwing a single securing screw into the corresponding bore provided with a thread, it is also possible here, after tightening the screw, to screw in and tighten a counterscrew.
Although in most cases one securing screw is sufficient, it is also possible to provide more than one securing screw. The two securing screws can in this case act on opposite-lying sides of the cone, for example. In order to reliably prevent the loosening of the securing screw or screws, a counterscrew can be screwed in behind the securing screw or screws.
The securing screw is positioned generally perpendicular to the axis of the conical bore and/or conical projection, i.e. the axis of the securing screw is 90.degree..+-.20.degree. from the axis of the conical bore and/or conical projection. It is further noted that the conical bore and conical projection can be replaced by complementary bores and projections having other tapered shapes which result in a substantially equivalent type of connection.